The field of the present invention is that of silicone compositions which are crosslinkable by activation, which can be used in particular to form an anti-adhesive coating or film for a fibrous or non-fibrous support, for example a support made of paper or the like, or alternatively made of natural or synthetic polymer.
More specifically, the invention relates to a process for the continuous coating of a travelling support strip (e.g. paper, fabric, polymer), in order to give it anti-adhesion properties, the said coating being carried out using a silicone composition based on polyorganosiloxane (POS).
The anti-adhesive POSs which are of interest in the context of the invention are of the type which are crosslinkable, by hydrosilylation or by dehydrogenopolycondensation, thermally or by a combination of heat and radiation (UV radiation or an electron beam).
Even more specifically the POSs considered are, for example, xe2x80x9cfast-curexe2x80x9d and xe2x80x9clow-temperature-curexe2x80x9d FC/LTC POSs.
The present also relates to the device and to the silicone composition involved in the continuous coating process, which constitutes its main subject.
Conventionally, anti-adhesive silicone compositions comprise at least one POS A, a crosslinking agent B which is a POS and a catalyst. These ternary compositions can only exist transiently in non-crosslinked form. The reason for this is that, irrespective of the crosslinking mechanism concerned: hydrosilylation or dehydrogenopolycondensation, POSs of SiH type placed in the presence, in the first case, of POSs of SiVi type and of platinum hydrosilylation catalyst, or placed in the presence, in the second case, of POSs of SiOH or SiOR type and of platinum or tin condensation catalyst crosslink relatively quickly. The speed of crosslinking depends in particular on the reaction temperature.
Moreover, it should be recalled that the POSs which make up these anti-adhesive compositions can be in non-crosslinked form, in pure form or in the form of crosslinkable liquid solutions or emulsions.
It follows therefrom that for the anti-adhesion treatments of supports (paper, fabric or polymer film), which consist in coating the surface of these supports with the abovementioned silicone compositions, it is imperative to apply and spread the said compositions when they are in non-crosslinked liquid form and thus entirely suited for use in the said operations.
Once the supports are coated with silicone composition, they are subjected to heating so as to accelerate their crosslinking.
In the light of the above, two difficulties can thus readily be foreseen, among others, which will arise when anti-adhesion treatments by silicone coating, e.g. of paper or of thermoplastic films, at the industrial speed and on the industrial scale are envisaged.
The first difficulty is associated with the fact that the heat-crosslinking silicone systems (using pure POSs, POSs in emulsion or in a solvent phase, etc.) which are usually used require crosslinking temperatures of between 100 and 200xc2x0 C. It is clear that at these temperatures, it is difficult, if not impossible, to envisage coating certain supports such as thermoplastic films, for instance polyethylene, polypropylene, PVC and, to a lesser extent, polyethylene glycol terephthalate. These silicone systems are also characterized by bath lifetimes BLTs of greater than or equal to 10 hours, and in practice between 12 and 24 hours.
The term BLT denotes the time required to double the dynamic viscosity measured at 30xc2x0 C. The dynamic viscosity can be measured using a Brookfield viscometer according to the indications of AFNOR standard NFT76102 of May 1982.
BLTsxe2x89xa710 hours give manufacturers a sufficient time margin to carry out the silicone coating at an industrial speed and on an industrial scale, while at the same time placing the constraints at a tolerable level. Unfortunately, the limitation imposed by the high crosslinking temperatures, as regards the variety of supports which can be treated by anti-adhesive silicone coating, constitutes a major handicap.
In addition, these high temperatures are detrimental to the product efficiency and financial viability of the coating treatments, on account of the high energy cost and long time required.
In an attempt to overcome these difficulties, solvent-free anti-adhesive silicone systems which can be thermally crosslinked and which claim an application performance known as LTC (low temperature crosslinking) have been proposed.
These thermal silicone systems crosslink at temperatures of between 60 and 110xc2x0 C. Unfortunately however, the corollary of this advantageous decrease in crosslinking temperature is a significant drop in the BLT which can fall quite considerably below 10 hours.
Such BLTs are incompatible with the current constraints of industrial use, which are intrinsic to the silicone anti-adhesion treatment by coating. The reason for this is that the preparation of the mixture forming the silicone composition is generally carried out in a batchwise manner. However, in these techniques, the supply of the coating head from the tank(s) in which the mixture is prepared necessarily involves periods of stagnation of the liquid silicone composition, before it is applied onto the support. It can thus readily be appreciated that with short BLTs, problems of gelation and of setting to a solid are liable to occur and to appreciably damage the quality of the coating.
Furthermore, the fact that one is constrained to prepare relatively large batches can entail considerable economic handicaps as soon as any incident liable to interrupt the process occurs on the coating line.
In summary, the industrial procedures for coating silicones are not entirely suited to LTC anti-adhesive silicone compositions, which correspond to the current best efforts as regards crosslinkable silicone systems for anti-adhesion. Their unsuitability is reflected in terms of cumbersome methodology, low production efficiency and financial viability, high cost and poor quality of the anti-adhesive coatings obtained.